Most of it depends on how you want to play the game. If you play it for the fun of the fishes, crossing fish and searching for Magic Fish, it sounds you are doing fine!

If you want LOTS of money ... then the second advertising research will help you raise prices on your fish. I never get the third food till I run out of things to buy.
The more money you have, the more Growth Hormone you can buy and so can breed the fish faster.
Or you can go about the game 'scientifically' and figure out the breeding patterns and 'how' to breed the fish and get the type you want. That's the direction I went, and went totally manic with the game.

It's not really a game to beat, as much as it's a game to have fun with and enjoy.

So if you're having fun ... keep feeding the fish!
